When a numerically controlled machine tool carries out a complicated machining of a workpiece, such as the machining of a pond shape, machining leaving an island shape projection, machining leaving an island shape projection in a pond shape and the like, this machining process requires not only the machining of a pond shape or an island shape but also a cutting between contours. For example, when an island shape projection is to be left in a pond, not only the inside wall contour of the pond but also the interior of the pond except for the island portion must be cut
Conventionally, an overall machining process for these portions is programmed manually or by using an automatic program creation device or the like, but such a manual programming of complicated shapes is time-consuming and prone to a very high error rate. Further, the automatic program creation device is expensive, it is difficult to use the device for correcting a program during machining, and the creation of a machining program requires a high degree of skill.